Methods and Systems for Injecting Liquid Into a Screw Compressor for Noise Suppression

ABSTRACT

A screw compressor for use in a chiller assembly includes cooperating screw rotors configured to increase the pressure of a vaporized refrigerant flowing through the compressor, a venturi tube arranged in a flow path of the refrigerant in the compressor downstream of the rotors, and an inlet port in fluid communication with a throat of the venturi tube and configured to deliver liquid refrigerant from a condenser of the chiller assembly to the flow path of the refrigerant in the compressor. The venturi tube is configured to cause a pressure drop in the refrigerant in the compressor. The liquid refrigerant delivered from the condenser reduces pulsations in the pressure of the refrigerant discharged from the compressor.

BACKGROUND

The present invention relates to suppressing noise generated in mechanical systems. In particular, the present invention relates to noise suppression in screw compressors used in commercial and industrial air conditioning and refrigeration systems.

The use of compression type water-cooled chillers is the most common method of cooling air in medium or large commercial, industrial and institutional buildings. Compression type water-cooled chillers are usually electrically driven, but may also be driven by a combustion engine or other power source. There are several types of compressors employed in water-cooled chillers. One common compressor is a screw compressor, which uses a rotary type positive displacement mechanism to compress a working fluid, such as a refrigerant.

Water cooled chillers used in air conditioning and refrigeration systems are required to meet stringent noise level requirements, such as those prescribed by the Occupational Safety and Health Association (OSHA). However, screw chillers have a tendency to generate significant noise during operation. The primary source of noise generated in these types of chillers is pressure pulsations originating from the compressor, which generates noise, as well as vibration of adjoining components. In addition to the screw compressor, there is a multitude of secondary sources of noise, such as the evaporator, the condenser, and the economizer.

Prior screw compressor designs have employed various devices and methods to suppress the noise generated by the compressor, such as mufflers and baffle plates arranged in the discharge chamber. Additionally, prior chillers have injected liquid refrigerant from the condenser into the gas refrigerant flow discharged from the compressor to suppress noise generated from pressure pulsations. However, under many operating conditions, these prior chiller designs have required a pressure application device, such as a pump, to compensate for a negative pressure differential between the condenser and the compressor. The addition of a pump, or other device, increases the cost and complexity of the system.

SUMMARY

A screw compressor for use in a chiller assembly includes cooperating screw rotors configured to increase the pressure of a vaporized refrigerant flowing through the compressor, a venturi tube arranged in a flow path of the refrigerant in the compressor downstream of the rotors, and an inlet port in fluid communication with a throat of the venturi tube and configured to deliver liquid refrigerant from a condenser of the chiller assembly to the flow path of the refrigerant in the compressor. The venturi tube is configured to cause a pressure drop in the refrigerant in the compressor. The liquid refrigerant delivered from the condenser reduces pulsations in the pressure of the refrigerant discharged from the compressor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a screw chiller assembly according to the present invention.

FIG. 2 is an axial section view of the screw compressor included in the chiller assembly of FIG. 1.

FIG. 3 is a schematic of the screw chiller assembly of FIG. 1 illustrating refrigerant flow through the system.

FIGS. 4A and 4B are schematics of two embodiments of the compressor from the chiller assembly of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 is a perspective view of screw chiller assembly 10 including screw compressor 12, variable frequency drive 14, condenser 16, and evaporator 18. In FIG. 1, the inlet of compressor 12 is fluidly connected to evaporator 18 and the outlet of compressor 12 is fluidly connected to condenser 16. Condenser 16 is fluidly connected to evaporator 18. Variable frequency drive 14 is mounted on condenser 16.

FIG. 2 is an axial section view of screw compressor 12 of FIG. 1, which compressor 12 includes compressor housing 20, drive screw 22, two opposed screws 24, 26, bearing housing 28, discharge housing 30, discharge chamber 32, discharge ports 34, and motor 48. Housing 20 receives central drive screw 22 and two opposed screws 24 and 26. Housing 20 is connected to motor 48, which is configured to drive screws 22, 24, 26. Bearing housing 28 receives screw bearings 28 a that facilitate low friction rotation of drive screw 22 and opposed screws 24, 26. Bearing housing 28 also receives compressed refrigerant from compression chambers 36 and delivers this compressed refrigerant through discharge ports 34 in the bearing housing 28 to discharge chamber 32 in discharge housing 30. The size of the discharge chamber 32 necks down with the inner peripheral surface 38 of the discharge housing 30.

FIG. 3 is a schematic of chiller assembly 10 illustrating flow of refrigerant through the system. Chiller assembly 10 is a closed loop system through which refrigerant is cycled in various states, such as liquid and vapor. As a somewhat arbitrary starting point in chiller assembly 10 of FIGS. 1-4, a low temperature, low pressure superheated gas refrigerant is sucked into screw compressor 12 through fluid conduit 42, such as a steel pipe, or other conduit from evaporator 18. Compressor 12 is driven by motor 48 under the control of variable frequency drive 14. Variable frequency drive 14 controls the frequency of the alternating current (AC) supplied to motor 48, thereby controlling the speed of motor 48 and the output of compressor 12. Refrigerant is sucked into compressor 12 through inlet ports 40, and compressed between screws 22, 24, and 22, 26 and carried towards discharge ports 34 in bearing housing 28. The compressed refrigerant enters discharge chamber 32 through discharge ports 34. After the refrigerant is compressed, the high temperature, high pressure superheated gas is discharged from compressor 12 through fluid conduit 42 to condenser 16. Chiller assembly 10 may also include an oil separator (not shown) between compressor 12 and condenser 16, which separates compressor lubricant from the refrigerant before delivering the refrigerant to condenser 16. In condenser 16, the gaseous refrigerant condenses into liquid as it gives up heat. The superheated gas refrigerant enters condenser 16 and is de-superheated, condensed, and sub-cooled through a heat exchange process with, for example, water flowing through condenser 16 to absorb heat. The liquid refrigerant is discharged from condenser 16 to metering device 44, which may convert the higher temperature, high pressure sub-cooled liquid to a low temperature saturated liquid-vapor mixture. The low temperature saturated liquid-vapor refrigerant mixture enters evaporator 18 from metering device 44 through fluid conduit 42. The low pressure environment in evaporator 18 causes the refrigerant to change states to a superheated gas and absorbs the required heat of vaporization from the chilled water, thus reducing the temperature of the water. The low pressure superheated gas is then drawn into the inlet of compressor 12 and the cycle is continually repeated. The chilled water is then circulated through a distribution system to cooling coils for providing air conditioning, or for other purposes.

Chiller assembly 10 may commonly be located in relatively close proximity to people and as such may be designed to suppress noise production and radiation as much as possible. Screw compressor 12 is a significant contributor to noise generation, because of pressure pulsations created when the refrigerant is compressed. Pressure pulsations in compressor 12 result from unsteady mass flux caused by the refrigerant compression process performed within compressor 12. The pressure pulsations in compressor 12 produce undesirable noise, which noise in turn is radiated from chiller assembly 10. Additionally, the pressure pulsations may generate mechanical vibrations in components of chiller assembly 10 such as piping, heat exchangers, or compressor housing 20 itself. Mechanical vibrations propagating through chiller assembly 10 may themselves result in further noise generation and radiation.

In order to suppress noise generated from the pressure pulsations in compressor 12, chiller assembly 10 includes liquid refrigerant conduit 46 shown in FIG. 3. Conduit 46 is configured to deliver liquid refrigerant from condenser 16 to the superheated gas refrigerant flow in compressor 12. In particular, conduit 46 is configured to deliver liquid refrigerant from condenser 16 to compressor 12 downstream of compression chambers 36 shown in FIG. 2. For example, conduit 46 may deliver liquid refrigerant to channels in bearing housing 28, which channels deliver the superheated gas refrigerant from compression chambers 36 to discharge chamber 32 through discharge ports 34. Noise in the gas refrigerant flow in compressor 12 is caused by pressure pulsations at frequencies in the audible range, which may range from approximately 20 to 20,000 Hz. Noise levels can be reduced by reducing the magnitude of such pressure pulsations. The objective of introducing liquid refrigerant from condenser 16 into gas refrigerant flow in compressor 12 is to reduce the strength of the pressure pulsations by transferring energy from the gas to liquid phase. Three mechanisms contribute to reduce pressure pulsations when liquid refrigerant droplets are injected into the gas refrigerant flow: a) viscous drag between liquid and gas refrigerant; b) heat transfer between liquid and gas refrigerant; and c) mass transfer from vaporization of liquid refrigerant to gas. Generally speaking, the magnitude of noise attenuation depends on the mass flow rate and droplet size of liquid refrigerant delivered from condenser 16. Noise suppression due to viscous drag and heat transfer are both functions of droplet size. Noise suppression due to mass transfer is a function of mass flow rate. Viscous drag and heat transfer are particularly effective to reduce noise at frequencies above 10,000 Hz, while vaporization, i.e. mass transfer, is effective at lower frequencies.

In order to deliver the liquid refrigerant from condenser 16 to the superheated gas refrigerant flow in compressor 12, the pressure in the condenser 16 must be greater than in the compressor 12. However, downstream of compression chambers 36 the superheated gas refrigerant often has a higher pressure than the pressure of the liquid refrigerant in condenser 16. Embodiments of the present invention therefore provide methods of and systems for inducing a pressure drop in the superheated gas refrigerant flow in compressor 12 sufficient to reduce the pressure in compressor 12 below the pressure in condenser 16 without the addition of work to the system.

FIGS. 4A and 4B are schematics of two embodiments of compressor 12 configured to induce a pressure drop in the superheated gas refrigerant flow discharged from compressor 12 through bearing housing 28 and discharge chamber 32. In FIGS. 4A and 4B, compressor 12 includes compressor housing 20, bearing housing 28, discharge housing 30, motor 48 and venturi tubes 50. Arranged in compressor housing 20 is compression chamber 36, which chamber 36 includes drive screw 22 and two opposed screws 24, 26 (shown in FIG. 2). Venturi tubes 50, also referred to as convergent-divergent or De Laval nozzles, include, in the direction of flow, a converging portion and diverging portion connected at a throat. The throat of venturi tubes 50 defines a location of minimum cross-sectional area and is in fluid communication with condenser 16 through conduit 46, which may be, for example, a steel pipe. In the embodiment of FIG. 4A, venturi tubes 50 are arranged in bearing housing 28 and are configured to direct refrigerant flow 52 from compressor 12 to discharge chamber 32 in discharge housing 30.

As refrigerant flow 52 passes through venturi tubes 50, the velocity of flow 52 increases while the pressure of flow 52 decreases. The throat of venturi tubes 50 defines not only the location of minimum cross-sectional area, but also the location of minimum pressure of refrigerant flow 52. Venturi tubes 50 thereby induce a pressure drop in refrigerant flow 52 being discharged from compressor 12 through bearing housing 28 and discharge chamber 32 to condenser 16. In embodiments of the present invention, venturi tube 50 is configured to induce a pressure drop in refrigerant flow 52 sufficient to reduce the pressure of flow 52 at the throat of venturi tube 50 below the pressure of liquid refrigerant directed through conduit 46 from condenser 16. Therefore the liquid refrigerant from condenser 16 used to suppress noise in compressor 12 may freely flow from condenser 16 to compressor 12 without adding work to the system, e.g., without the use of a pressure applicator like a pump.

In some applications, space constraints in compressor 12 may not permit venturi tubes 50 to be disposed in bearing housing 28. In an alternative embodiment (FIG. 4B), venturi tubes 50 are arranged within discharge chamber 32 of discharge housing 30. In the embodiment of FIG. 4B, refrigerant flow 52 passes through bearing housing 28 into venturi tubes 50 in discharge chamber 32 through discharge ports 34. A pressure drop is induced in refrigerant flow 52 as the refrigerant passes through venturi tubes 50, which pressure drop enables liquid refrigerant from condenser 16 to freely flow from condenser 16 through conduit 46 to compressor 12 without adding work to the system.

Embodiments of the present invention provide methods of and systems for inducing a pressure drop in the superheated gas refrigerant flow in a screw compressor of a chiller assembly sufficient to reduce the pressure in the compressor below the pressure in a condenser without the addition of work to the system. Inducing a pressure drop in the compressor refrigerant flow enables liquid refrigerant from the condenser to freely flow to the compressor without the use of a pressure application device, such as a pump. Embodiments of the present invention thereby suppress noise generated from pressure pulsations in the screw compressor by injecting liquid from the condenser into the gas refrigerant flow in the compressor without significantly increasing the cost and complexity of the chiller assembly.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. 

1. A screw compressor for use in a chiller assembly, the compressor comprising: a plurality of cooperating screw rotors configured to increase the pressure of a vaporized refrigerant flowing through the compressor; a first venturi tube arranged in a first flow path of the refrigerant in the compressor downstream of the rotors for causing a pressure drop in the refrigerant; and a first inlet port in fluid communication with a throat of the first venturi tube and configured to deliver liquid refrigerant from a condenser of the chiller assembly to the flow path of the refrigerant in the compressor for reducing pulsations in the pressure of the refrigerant discharged from the compressor.
 2. The compressor of claim 1, wherein the first venturi tube is located in a bearing housing of the compressor.
 3. The compressor of claim 1, wherein the first venturi tube is located in a discharge housing of the compressor.
 4. The compressor of claim 1 further comprising: a second venturi tube arranged in a second flow path of the refrigerant in the compressor; and a second inlet port in fluid communication with a throat of the second venturi tube and configured to deliver liquid refrigerant from the condenser to the second flow path of the refrigerant in the compressor.
 5. The compressor of claim 1, wherein the first venturi tube reduces the pressure of the refrigerant in the compressor below a pressure of the refrigerant in the condenser.
 6. A chiller assembly comprising: a screw compressor; a condenser coupled to the screw compressor; a first venturi tube arranged in a first flow path of a refrigerant passing through the compressor, wherein the first venturi tube comprises a convergent portion connected to a divergent portion at a throat; and a first conduit coupled between the throat of the first venturi tube and the condenser and configured to deliver liquid refrigerant from the condenser to the first flow path of the refrigerant in the compressor for reducing pulsations in the compressed refrigerant exiting the compressor.
 7. The assembly of claim 6, wherein the screw compressor comprises: a plurality of cooperating screw rotors configured to increase a pressure of the refrigerant flowing through the compressor, wherein the first venturi tube is arranged downstream of the screw rotors.
 8. The assembly of claim 7, wherein the first venturi tube is located in a bearing housing of the compressor.
 9. The assembly of claim 7, wherein the first venturi tube of the discharge chamber is located in a discharge housing of the compressor.
 10. The assembly of claim 6 further comprising: a second venturi tube arranged in a second flow path of the refrigerant in the compressor, wherein the second venturi tube comprises a convergent portion connected to a divergent portion at a throat; and a second conduit coupled between the throat of the second venturi tube and the condenser is configured to deliver liquid refrigerant from the condenser to the second flow path of the refrigerant in the compressor.
 11. The assembly of claim 6, wherein the first venturi tube reduces a pressure of the refrigerant in the compressor below a pressure of the refrigerant in the condenser.
 12. A screw compressor for use in a chiller assembly, the compressor comprising: a screw rotor bearing housing; a first venturi tube disposed in the bearing housing and arranged in a first flow path of a refrigerant carried through the bearing housing for decreasing the pressure of the refrigerant; and a first inlet port in fluid communication with a throat of the first venturi tube and configured to deliver liquid refrigerant from a condenser of the chiller assembly to the first flow path of the refrigerant in the bearing housing.
 13. The compressor of claim 12 further comprising: a second venturi tube disposed in the bearing housing and arranged in a second flow path of the refrigerant carried through the bearing housing; and a second inlet port in fluid communication with a throat of the second venturi tube and configured to deliver liquid refrigerant from the condenser to the second flow path of the refrigerant in the bearing housing.
 14. The compressor of claim 12, wherein the venturi tube reduces a pressure of the refrigerant in the bearing housing below a pressure of the refrigerant in the condenser.
 15. A screw compressor for use in a chiller assembly, the compressor comprising: a discharge housing; a first venturi tube disposed in the discharge housing and arranged in a first flow path of the refrigerant carried through the discharge housing for decreasing the pressure of the refrigerant; and a first inlet port in fluid communication with the throat of the venturi tube and configured to deliver liquid refrigerant from a condenser of the chiller assembly to the first flow path of the refrigerant in the discharge housing.
 16. The compressor of claim 15 further comprising: a second venturi tube disposed in the discharge housing and arranged in a second flow path of the refrigerant carried through the discharge housing; and a second inlet port in fluid communication with a throat of the second venturi tube and configured to deliver liquid refrigerant from the condenser to the second flow path of the refrigerant in the discharge housing.
 17. The compressor of claim 15, wherein the first venturi tube reduces a pressure of the refrigerant in the discharge housing below a pressure of the refrigerant in the condenser.
 18. A method of suppressing noise in a screw compressor of a chiller assembly, the method comprising: introducing a liquid refrigerant from a condenser of the chiller assembly into a compressed gas refrigerant flowing through the screw compressor to reduce pulsations in the refrigerant; and reducing, without adding work, a pressure of the gas refrigerant in the compressor below a pressure of the liquid refrigerant in the condenser to facilitate introduction of the liquid refrigerant into the gas refrigerant.
 19. The method of claim 18, wherein the pressure of the gas refrigerant is reduced by passing the gas refrigerant through one or more venturi tubes.
 20. The method of claim 18, wherein the pressure of the gas refrigerant is reduced in one of a bearing housing or a discharge housing of the compressor. 